Phosphors are usually expensive as compared with ordinary chemical materials, since they have a special property to convert invisible radiant energy to a visible light. In recent years, substantial amounts of rare earth elements or noble metal elements have been used as raw materials to satisfy various properties. There have been various restrictions in their use attributable to the costs, and there has been a difficulty that the most suitable phosphor for a particular purpose is not always available because of such restrictions. Therefore, a reduction of the costs for such phosphors is strongly desired.
On the other hand, as a technique for this purpose, which is close to the present invention, it is disclosed, for example, in U.K. Patent No. 603,326 that in a fluorescent lamp, the amount of a phosphor material can be substantially reduced by providing a layer of a substance having excellent ultraviolet reflectance between the support and the phosphor. Further, as an improvement of this technique, U.S. Pat. No. 4,069,441 discloses an electric gas discharge lamp wherein a layer consisting of a mixture of a phosphor and a white material having a small absorption for ultraviolet rays, such as barium sulfate, calcium pyrophosphate or magnesium oxide, is provided on a glass tube, and a layer consisting solely of a phosphor is laminated thereon, and the layer consisting of the mixture is formed so that the ratio of the amount of the white material to the amount of the phosphor increases towards the glass tube. A related technique is also disclosed in Japanese Unexamined Patent Publication No. 128452/1982. Mixtures obtained by simply mixing phosphors with white pigments depending upon the particular specific applications (such as for X-ray image conversion screens) are disclosed in e.g. U.S. Pat. No. 4,039,840, Japanese Examined Patent Publication No. 5478/1981 and Japanese Unexamined Patent Publication No. 146447/1980. Furthermore, alkaline earth metal sulfate phosphors (i.e. phosphors wherein alkaline earth metal sulfates are used as host materials) are also known, for example, from Japanese Examined Patent Publications No. 37069/1979 and No. 50832/1982. These phosphors contain lead or bivalent europium as activator, and emit ultraviolet or blue colors under excitation with radiant energy.
The above-mentioned mixture of the phosphor and the white material has a difficulty such that the decrease in the luminance is substantially large from the practical point of view, and in order to minimize this decrease or in order to provide other effects, cumbersome means such as the formation of the special layer structure as shown in the above publications,, is required. In many cases, the amount of the use of the mixture is restricted to a level of not higher than a few percent by weight.
As mentioned above, none of the conventional techniques is capable of substantially reducing the costs for such luminescent phosphor materials without no substantial reduction of the luminance of the luminescent phosphor materials.